1. Field of the Invention
This invention relates to fiber lasers, and more particularly to a multiple output fiber laser system with passive frequency control and a method for making the same.
2. Description of the Related Art
Target detection systems, such as a laser radar (LADAR), typically use a pulsed optical signal beam from a pulsed laser to track moving targets. One such LADAR system is described in co-pending U.S. patent application Ser. No. 08/369,023 filed on 5 Jan. 1995, entitled "RANDOM PULSE BURST RANGE-RESOLVED DOPPLER LASER RADAR" by Joseph N. Paranto et al., and assigned to Hughes Aircraft Company, the assignee of the present invention.
The pulsed signal beam reflects from a target and is collected and sent to a heterodyne receiver, which mixes reflected pulses with a local oscillator beam, typically an optical beam from a continuous-wave (CW) laser. For effective heterodyne detection of the reflected signal beam, the frequency relationship between it and the local oscillator beam must remain constant over time. This creates a problem because the local oscillator and pulsed signal beams are created by two separate lasers whose operating frequencies drift over time due to, among other factors, changes in environmental conditions (i.e., temperature, stress, strain).
Prior systems overcome this problem by utilizing tunable single-longitudinal mode (single-mode) solid state lasers to generate the local oscillator beam, such as those described in B. E. A. Saleh, et al., "Fundamentals of Photonics," John Wiley & Sons, 1991, pp. 518-519. The local oscillator beam is actively tuned to track any drifts in the frequency of the pulsed signal beam. These tunable lasers are very expensive and they increase the total size and complexity of the LADAR system due to the associated tuning and feedback circuits that are required.